Certain laminated glazing units, such as motor vehicle windshields and the like, are provided with a substantially transparent, electrically conductive coating at a laminated interface between adjacent plys. Such coatings are known for use, for example, to reduce the amount of infrared light transmitted through the glazing unit. Architectural and motor vehicle windows are provided with such coatings to reduce the amount of heat energy passing through the window to reduce the air conditioning load. Such coatings, commonly referred to as solar load reduction or SLR coatings, typically comprise a film stack wherein one or more films of electrically conductive metal alternate with films of dielectric material, such as metal oxide. The individual films within the film stack may be deposited onto a substrate, preferably a substantially rigid and inextensible ply of the glazing unit, such as an exterior glass ply, by pyrolytic deposition, sputter coating, or other technique known to those skilled in the art.
The same film stack technology also is used for electrically heating glazing units. That is, substantially transparent film stacks comprising electrically conductive metal films alternating with dielectric films can be connected to a power source, such as a motor vehicle battery, generator or alternator system, by suitable circuitry. The sheet resistance of the film stack is selected to provide a desirable rate of heat generation in response to electric current flow through the film stack. Motor vehicle windshields presently manufactured with such electrical heating means are known to employ a sputtered coating comprising a substantially transparent, electrically conductive film of silver metal sandwiched between films of zinc oxide.
Lamination of the surface of the glass ply (or other glazing unit ply) carrying the aforesaid film stack to a laminating material such as polyvinyl butyral (PVB) or other flexible, polymeric laminating material, is found to provide good interfacial adhesion. It has been found, however, that interfacial adhesion may be reduced by prolonged exposure to UV radiation, such as the UV component of sunlight. This is found to be true particularly in the case of PVB and other flexible polymeric laminating materials having an alcoholic hydroxyl group (R-OH). For purposes of improving the durability of glazing units having a laminated interface between a surface of a ply coated with a substantially transparent, electrically conductive film stack and a ply of flexible, polymeric laminating material, there is a need to improve the durability of the adhesion between such film stack and the laminating material against prolonged exposure to UV radiation. Improved interfacial adhesion is taught in U.S. Pat. No. 4,844,985 to Pharms et al, assigned to Ford Motor Company, wherein a barrier or adhesion layer of chromium oxide complex is taught. Specifically, a thin layer of chromium oxide complex is deposited over a zinc oxide dielectric layer in a film stack comprising alternating layers of silver and zinc oxide. This enhances the durability of the interfacial adhesion to PVB and like laminating materials. The use of a chromium oxide complex barrier or adhesion layer, however, imparts a certain degree of "color" to the glazing unit due to its refraction and absorbance properties. A substantial need exists, therefore, to improve interfacial adhesion durability while imparting less color to the glazing unit than does chromium oxide complex.
It is an object of the present invention to improve the durability of laminated glazing units by improving the durability of such interfacial adhesion against prolonged exposure to UV radiation. Additional objects and advantages of the invention will be understood from the following disclosure thereof and detailed description of various preferred embodiments.